The present invention relates to a simultaneously ground type bearing body suitable for use in a table saddle of a machine tool, slide surfaces in a machining center and other transfer devices which are adapted to make reciprocatory sliding motion while carrying heavy loads. The invention also pertains to a limited sliding bearing unit employing the simultaneously ground type bearing body.
In a typical conventional limited sliding bearing unit, a pair of recesses 105, 106 are formed on the right and left (as viewed in FIG. 9) of a central cavity 102 of a bearing body 101 by means of broaching. In addition, two ball rolling surfaces 107, 108 are formed on both sides of the recess 105, while two ball rolling surfaces 109, 110 are formed on both sides of the recess 106. Then, the ball rolling surfaces 107, 108, 109, 110 are hardened to produce the bearing body 101. Since the ball rolling surfaces 107, 108, 109, 110 may be distorted owing to heating in the hardening, however, the ball rolling surfaces are generally finished by means of a grinding wheel to obtain a required dimensional accuracy.
The conventional grinding for finish is such that, as shown in FIG. 9, the ball rolling surfaces 108, 110 closer to the upper sides of the right and left skirts 103, 104 are first ground successively by means of a grinding wheel G which revolves with its axis of rotation placed vertically, and then, the ball rolling surfaces 107, 109 closer to the lower sides of the right and left skirts 103, 104 are successively ground by means of a grinding wheel G which revolves with its axis of rotation placed horizontally. The prior art, however, is not suitable for mass production, since four steps are required for the grinding operation. Moreover, owing to the individual grinding for each of the four ball rolling surfaces, it is difficult to uniformly grind all the ball rolling surfaces to close tolerances. In consequence, in order to obtain required dimensions with high accuracy, an impractically long time is needed for the grinding operation, which constitutes one of factors in a high cost. If a required dimensional accuracy cannot satisfactorily be obtained, e.g., if the infeed of the grinding wheel is excessive, there will be play between the ball rolling surfaces on the bearing body and the corresponding ball rolling surfaces formed on the associated track shaft when balls are recirculated through the passageways defined by these ball rolling surfaces, resulting in various problems, such as a poor torque transmission, the generation of noise and the inclination of the bearing body. If the infeed of the grinding wheel is insufficient, on the other hand, it becomes difficult to insert balls into the space defined between the ball rolling surfaces on the bearing body and the corresponding ball rolling surfaces on the track shaft. In addition, after the insertion of the balls, the movement of the balls becomes stiff due to an increase in resistance to sliding, which makes it impossible to obtain a smooth traveling of the bearing unit disadvantageously.